Development of a Compact Radiography Accelerator Using Dielectric Wall Accelerator Technology Page: 5 of 6
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insulator structures of length d. Others have also
suggested a resultant modification to the local electrical
field that modifies the net trajectory of the electrons,
responsible for breakdown, away from the surface.
Regardless of the mechanism, these structure showed
robust performance under arced conditions and ranged in
surface flashover fields from >100 MV/m for 1-3 ns
pulses and >100 kV/cm for multi-microsecond pulses to
DC. Materials varied from fused silica for UHV
applications to plastics treated to reduce the outgassing
rates to 2.3 x 10-9 (T-/s-cm2).
B. Pulse forming Lines and Dielectric Materials
The pulse forming lines for the DWA are fully
integrated into each accelerator cell and result in a very
compact system. Work was performed for two
geometries: (1) the asymmetric Blumlein and (2) the
symmetric Blumlein
The original asymmetric Blumlein, required a large
radial extent for a given pulse width and generated a more
triangular pulse shape; energy regulation for proper beam
transport was difficult to obtain. Further, because of the
dissimilar dielectrics, full energy transfer to the load was
not possible. The symmetric Blumlein, however, allows
full energy transfer. In addition we were able to
implement constant impedance pulse forming lines for the
generation of a more trapezoidal pulse shape with a
flattop. To minimize the radial extent of the lines, a
serpentine pattern was used. In this configuration, layer-
to-layer coupling could occur, but using full 3-d, time-
domain electromagnetic field solvers modified
specifically for pulsed power applications, allowed us to
optimize our designs with a minimum amount of
prototyping.
A significant effort focused on the development of
engineered dielectrics for the DWA. Initially, pure
ceramics were pursued for their high dielectric constant
and moderate field gradients. Fabrication was costly and
part yield was low. Development of castable composite
dielectrics, however, allowed ease of fabrication through a
casting process [7]. In addition, tailoring the permittivity
to a system specific requirement was possible. Values for
the specific energy density as a function of dielectric
strength for much of this work are shown in Figure 4.
The most notable point, at 250 MV/m and specific energy
density of >12 J/cm3, were taken from ten tests conducted
with ball electrodes. Enhancement was not taken into
account, hence, these values are conservative. Standard
deviation of the breakdown electric field was <0.2
MV/cm or <1% of the applied field.
This latter technology is based on a high dielectric
constant nano-composite comprised of a polymer resin
system and nano-size inorganic particles. The material
allows for dielectric moldings with complex shapes or
effective encapsulation of electrodes. The
polymer/particle slurry does not contain solvents or
volatiles and can be net shape formed in 3-dimensional
structures.Component fabrication is achieved by casting the high
viscosity slurry and thermally curing the resin system at
relatively low temperatures (~100 C). The physical
strength and toughness of the cured dielectric provided a
high level of structural integrity. Successful component
fabrication is dependent on proper conductor designs and
material processing steps including particle dispersion,
casting method, thermal and pressure profiles.
The performance of the nano-composite dielectric is
achieved through a combination of inorganic powders,
dispersants, polymers and cure agents. The polymer
system provides the forming characteristics required for
encapsulation while the inorganic particles provide an
increase in dielectric constant. Proper incorporation of
the nano-size particles in the polymer allows for the
dielectric constant of the polymer to be increased from
approximately 3 to 55. Further, the voltage stress
capability of the polymer is not compromised when using
the nano-size powders. The intrinsic dielectric strength of
the dielectric has been defined at 250 MV/m.c
a
d
s=
a,
N
QJ
Ni15.0
10.0
5.0
0.00 50 100 150 200
Dielectric Strength, MV/m250 300
Figure 4. Specific energy density of pulse forming line
dielectrics pursued for the DWA
B. Gas Discharge and Photoconductive
Switching
Two switching configurations were tested using RT
Duroid as the pulse forming line material. Specific
energy density is comparable to most plastics. The first
configuration allowed observation of the optical signature
of ten laser triggered gas switches integrated into a ten-
stage Blumlein module. These optical signals were
captured on a streak camera and the leading edge was
used to measure switching simultaneity (fig. 5). The solid
vertical line depicts propagation time for the laser pulse.
From these signatures, jitter was determined to be <1 ns.
The second configuration used UV-pre-illuminated gas
switches. Modifications were made to ensure a low
inductance-switching configuration. From this effort we
were able to demonstrate a 3-4 ns risetime and ~A ns
simultaneity on the switch closure time. Output from this2
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Sampayan, S.; Caporaso, G.; Chen, Y.; Hawkins, S.; Holmes, C.; Krogh, M. et al. Development of a Compact Radiography Accelerator Using Dielectric Wall Accelerator Technology, article, June 2, 2005; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc878520/m1/5/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.